[BUGFIX] Hook call modifyDBRow in ContentContentObject

Replace $this->TYPO3_CONF_VARS with $GLOBALS['TYPO3_CONF_VARS']

Fixes: #44416
Releases: 6.0, 6.1, 4.7, 4.6, 4.5
Change-Id: I1f2b6a2479cc879c2a9144d03e176b74d895d52f
Reviewed-on: https://review.typo3.org/17964
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn